Draft changelog: add Sep 9, 2026 API updates

[mintlify]

Summary

Adds three changelog entries covering everything merged since the Sep 1, 2026 entry.

Changes

• New feature: Enterprise-only Block and Unblock user endpoints (POST /2/users/{id}/blocking, DELETE /2/users/{source_user_id}/blocking/{target_user_id}).
• New feature: subscriptions.subscribe and subscriptions.unsubscribe events for the X Activity API.
• Update: new broadcast.read and broadcast.write OAuth 2.0 scopes for live broadcasts.

Context

Based on merged PRs #404, #402, and #350 (all merged Sep 9, 2026).

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